This invention relates to a method and apparatus for recovering digitized data from a radio channel. More specifically the invention relates to synchronization and tracking of synchronous data, typically in packet format, primarily for applications where information is conveyed via radio or similar free-space-like channel. In particular, the invention relates to data processing after a synchronous signal has been demodulated to a baseband signal and filtered in preparation for information extraction. The invention is applicable to any modulation method in which a modulated signal is demodulated to an analog baseband signal for information recovery.
A number of problems have been identified in connection with the acquisition of a signal after demodulation to baseband. Due to typical frequency errors in an FM system, for example, d.c. errors are introduced into the baseband signal resulting in a d.c. level offset. In addition, it is necessary to acquire or establish the relative timing of synchronous symbols, i.e., symbol timing, so that sampling of the symbols can be synchronized for optimum data extraction. Such clock synchronization is commonly acquired during the transmission of a preamble stream of symbols. In other words, the purpose of clock synchronization is to establish a clock which can be used for controlling the sampling of subsequent symbols.
Processes are known for converting an analog signal to a digital signal for further information recovery and for signal processing using for example samples of digital values representing analog amplitude levels. What is needed is a technique for acquiring symbol timing and correcting for d.c. offset independent of one another after an analog signal has been digitized for further information recovery.